The necessity of internally screened telephone cables has been adequately described in U.S. Pat. No. 3,803,340 which was awarded to General Cable Corporation on Apr. 9, 1974. The construction disclosed in the patent met established telephone industry standards for 24-channel PCM carrier transmission at 772 kHz; and the near end crosstalk isolation margins were better than those of screened cables of other telephone cable manufacturers at that time.
With the passage of time, however, technology advanced such that the original screened cables no longer provided an adequate margin of performance for expanded capacity PCM systems. In addition, economic considerations became of prime concern. Both low cost and high performance had to go hand in hand. Furthermore, as time progressed, certain weaknesses in the original designs became evident. An enumeration of such deficiencies, whether due to advancements of the art, the concerns of cost or just inadequacies of the original design include:
(a) The established practice of many telephone operating companies of electrically "floating" (not grounding) the screen while grounding the overall shield. This placed a heavy demand upon the screen shielding efficiency.
(b) The foregoing situation also imposed severe voltage withstand requirements on the screen insulant layers. Because of this condition, certain manufacturers of telephone cable imposed the requirement that these insulant layers had to withstand a 10 kV-DC potential level.
(c) With the continued expansion of channel capacity, and the corresponding increased frequency bandwidth, increased shielding efficiency was required of the screening component. At a Nyquist frequency of 1.576 MHz, for example the original "D" screen design (referenced patent) offered but marginal compliance. In this case compliance refers to the tentative industrial standard of 80 db worst case Power Sum Near End Crosstalk.
(d) The complexity of the presently employed screening tapes, and the associated high costs, have significantly increased the attractiveness of the prior art screened cable system with respect to two cable operation.
(e) The use of shields and/or screens of the prior art which rely on overlap seams which occur on or about the shield circumference tend to impair the mechanical strength of the composite cable, and in addition have a noticeable effect upon shield life because of the effects of corrosion.
The invention of this specification overcomes these problems by changes in the screens that shield the groups of conductors from one another and from the outside with more efficient screening and obtain acceptable limits of near end crosstalk. At the same time, the changes in the shielding screens obtain structurally stronger communication cables and protect uncoated edges of aluminum screening tapes from corrosion, thereby increasing the useful life of the cable.
Revisions in the geometry of the shielding screens also protect the conductors in the cable from water which enters the cable through breaks or imperfections in the outside jacket of the cable.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.